The present disclosure relates generally to superconductive magnets, and more particularly to a superconductive magnet having a cryocooler coldhead.
Magnets include resistive and superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics. Known superconductive magnets include liquid-helium-cooled, cryocooler-cooled, and hybrid-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in solid conduction thermal contact with the thermal shield, and having its second stage in solid conduction thermal contact with the superconductive main coil. A liquid-helium-cooled magnet typically also includes a liquid-helium vessel surrounding the superconductive main coil with the thermal shield surrounding the liquid-helium vessel. A hybrid-cooled magnet uses both liquid helium (or other liquid or gaseous cryogen) and a cryocooler coldhead, and includes designs wherein the first stage of the cryocooler coldhead is in solid conduction thermal contact with the thermal shield and wherein the second stage of the cryocooler coldhead penetrates the liquid-helium vessel to recondense “boiled-off” helium. Superconducting magnets which recondense the helium gas back to liquid helium are often referred to as zero boiloff (ZBO) magnets.
Known resistive and superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a constant magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet's bore where the object to be imaged is placed.
Open magnets, including “C” shape and support-post magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design.
Cryogens such as liquid helium, however, are not abundant and therefore can significantly impact the cost of operation of the MRI system. As a result, a zero boil-off design has far better advantage over a lower boil-off design, since the former design consumes no helium during normal operation. In the current zero boil-off magnet design, the magnet assembly only has a single radiation thermal shield which is wrapped by multiple layers of superinsulation. A temperature on the thermal shield, depending on the thermal shield conductance thereof, is about 45° K. to 70° K. The radiation heat load from the thermal shield to the helium vessel attributes to 50% of the total head load.
However, when the cryocooler coldhead extending through a penetration to the liquid-helium vessel is not operational due to power off, coldhead failure or transportation, the coldhead acts as a heat source and adds significant heat into the cryostat. The temperature on the single radiation thermal shield on such a zero boil-off design will climb up to about 100° K. to about 150° K. The increase in temperature depends on the thermal shield conductance, conductance of copper braids between a coldhead sleeve assembly and the thermal shield, and the radiation heat between the coldhead and the helium vessel, which attributes to most of the total head load and thus boil-off of the helium at a rate of about 1.4 liter/w.
Accordingly, there is need in the art for an apparatus and method to reduce radiation heat load between the thermal shield and the helium vessel, conduction heat from the coldhead to the thermal shield, and conduction heat load between the penetration and the thermal shield when the coldhead is not operational.